While the present invention might apply to a variety of receivers, it is specifically designed as a transponder for use in satellite communications. Specifically, when the receiver receives a signal of the correct frequency, the frequency of the transmitter is controlled so that it is phase locked with the signal received by the receiver.
More specifically, in satellite communications transponders are generaly utilized which are referred to as S-band transponders. The prior art receivers of these S-band transponders operate either in the SGLS (DOD) frequency range of 1763.721 to 1839.795 mHz or in the STDN (NASA) frequency range of 2028.1 to 2115.6 mHz. It should be noted that S-band transponders transmit in the same frequency band whether in SGLS or STDN mode and, therefore, no modification to the transmitter frequency scheme is necessary or desirable in order to implement the dual mode function. To convert the prior art receivers to dual mode, so they will receive either the SGLS mode or the STDN mode, requires additional duplicate circuts, such as two voltage controlled oscillators or two LO multipliers. The present dual mode receiver provides the dual mode function with no increase in complexity of the receiver.